Metal powders have gained great importance with the advances of powder metallurgy. The production processes include, for example: grinding brittle metals or alloys, spraying of melts, the reduction of powdery oxides, thermal decomposition or precipitation of organometallic compounds, and chemical and electrolytic deposition. The various processes produce powders with very different properties. In this connection, besides the properties of the material, the morphological powder properties (particle shape, particle size distribution) play a large role in the processing steps of powder preparation, shaping and consolidation. Thus, the latter also have a great influence on the residual porosity and the surface composition as well as on the structure of the final product.
Electrolytically produced powders are often comprised of dendritically grown crystals. Powders formed on stationary electrodes show, depending on electrolysis conditions, particle sizes between 300 and 1 .mu.m.
The powdery precipitate on the cathode is formed in the electrolytic process under conditions which are opposite to those of electrolytic plate formation. As a rule, the precipitates crystallize in a powdery manner at high current densities, low metal ion concentrations and low bath temperatures. To intensify the transport of material, oscillating or rotating electrodes are used which simultaneously foster the detachment of the powder deposited on the electrode. The powdery precipitate detached or to be brushed off from the electrode is collected either at the bottom of electrolytic cells or in an organic medium underneath the electrolyte (two-phase bath).
In recent years, noble metal alloy powders have also received attention because of their advantageous physical-chemical properties. Thus, for example, silver-palladium alloy powders were developed for dental prosthetic applications. Other possibilities of use can be foreseen in the field of electronics and in the chemical industry.
A process for electrolytic production of pourable powders from noble metals, especially from platinum, palladium or gold, is known, for example, from DD-PS 139 605. According to the publication, powders of defined particle size are said to be producible by electrolytic methods if the precipitation is performed with solutions of platinum metal hydrochloric acids and gold hydrochloric acids in the diffusion limiting current range, i.e., in the range between solid precipitation and hydrogen formation. In particular, the particle size in the previously described process is said to be able to be influenced by a variation of the concentration, the temperature and the pH.
But even though the electrolytic precipitation of pourable powders made of noble metals, preferably platinum, palladium, rhodium, gold and their alloys, is indicated in the patent specification as a field of use of the invention, the disclosed technical teaching exclusively relates to the precipitation of pure metals. No reference is made to the electrolytic precipitation of alloyed metallic powders.
However, a process for electrolytic production of alloyed AgPd powders is described in the article "Electrolytic Preparation of Fine PdAg Powders of Any Given Composition," M. I. Kalinin Leningrad Engineering Institute, translated from Poroshkovaya Metallurgiya, No. 6 (126), pp. 6-10, June 1973.
The authors report on systematic studies on the influence of the electrolysis parameters of bath composition, bath temperature and current density in the powder precipitation range in the chemical and crystallographic composition as well as the particle size and morphology of the precipitated powder and indicate value ranges for the electrolysis parameters as a result of their studies for the AgPd system, within which a precipitation of true alloy powders with predetermined composition is said to be possible.
A drawback of this known process is that the indicated, empirically determined ranges of values for the electrolysis parameters are very narrow and relate exclusively to the AgPd system as well as to a specific electrolytic bath. A transfer of the obtained results to other electrolytic baths or other alloying systems is not possible.